I would put money that its the Rotors. Particularly the passenger side. The last time I swapped my passenger rotor was .040 off in one direction, and drivers side was .002 in one direction .003 in the other.
Owner reviews for brake discs
We run a Stoptech BBK on the BMW 335 car that I race in endurance races (4, 9 or 12 hour) and we run either slotted or plain rotors, but never run cross-drilled. Our rotors last longer and we don't have the cracking issues associated with cross-drilled.
I have drilled and slotted rotors on my MK7. I have never had any warping or cracking issues with them, or one any other car that I've ever had with them. Performance wise I've never noticed a difference between stock or drilled/slotted rotors except for in wet conditions. When it's wet out break feel is MUCH better with drilled/slotted rotors.
I've been running slotted and cross-drilled rotors on my A4 for 5 years. I've never had one crack, but I don't autocross. For my latest set I looked for drilled, but not slotted, rotors because my last set buffeted at high-speed braking, which feels shaky and unstable, and I read that was because of the slots.
I was doing some more research, and found out that Alcon has replacement rotors for the front, albeit Price is high(on par with RacingBrake's price but overall better quality i'm sure) I'm sure it is worth it, which will also allow for the partially cheaper rear matching Alcon Big rotor kit for the golf R to be utilized.
+1 for stoptech st-40s, they're slotted 328mm rotors and 4 pot calipers. Car brakes really nice. Kept the stock rears, although had to replace calipers at about 120k as they were starting to seize. If the car sits for a few days, the rust builds up (I assume in the slots) and makes noise . Goes away after about 30-40 miles. Not sure if drilled would be better. I definitely don't regret it, but modulation from the 4-pot isn't improved the way I thought it'd be. For the same price or just a tad more I could've gone with R32/GolfR/etc. front and rear, I'd probably do that if I were to do it again.
Picked these up because my ultimate goal was to do something OEM+. These were drilled with a very OEM like coating and when the calipers were painted blue it looked identical to those R32/R36 brake set up. Although I only had them for a short period of time, I've previously used e-Line on other cars and had no problems just like the Premium one.
I just changed the front pads/rotors on my '04 325i to Zimmerman with the OEM Textar pads. Seem to work same as before, no complaints from me. They are dusty like people say, but I don't care too much about that.
Brakes: (front) StopTech 13.5” slotted Frodo, 944 Porsche pads, (rear) VR6 disc from 1996 Golf
The rear brakes will be an absolute nightmare. There is not a lot of room. As far as I am aware, MK6/MKV R/R32 brakes are the same, so... prepare yourself for a miserable time doing the rear rotors. The rear pads are fine. The rotors suck ass.
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